Remove mmap code from sfio

Remove mmap code from sfio

There are multiple reasons to remove this code:
- The define that guards all the mmap code (_mmap_worthy) is rarely set.
  When building local or on Travis, the define isn't set.
  On Windows, it is never set.
- If a 'binary compatibility package' of sfio is build (when _SFBINARY_H
  is defined), `_sys_mann` is `undef`ed, excluding mmap code from the build.

Instead of keeping the code that is rarely included, we can reduce code
complexity by removing it.